How big can a user agent string get?

If you were going to store a user agent in a database, how large would you accomdate for?

I found this technet article which recommends keeping UA under 200. It doesn't look like this is defined in the HTTP specification at least not that I found. My UA is already 149 characters, and it seems like each version of .NET will be adding to it.

I know I can parse the string out and break it down but I'd rather not.

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EDIT
Based on this Blog IE9 will be changing to send the short UA string. This is a good change.

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HTTP specification does not limit length of headers at all. However web-servers do limit header size they accept, throwing 413 Entity Too Large if it exceeds.

Depending on web-server and their settings these limits vary from 4KB to 64KB (total for all headers).

My take on this:

• Use a dedicated table to store only UserAgents (normalize it)
• In your related tables, store an Foreign Key value to point back to the UserAgent auto-increment primary key field
• Store the actual UserAgent string in a TEXT field and care not about the length
• Have another UNIQUE BINARY(32) (or 64, or 128 depending on your hash length) and hash the UserAgent

Some UA strings can get obscenely long. This should spare you the worries. Also enforce a maximum length in your INSERTer to keep UA strings it under 4KB. Unless someone is emailing you in the user-agent, it should not go over that length.

Noticed something like this in our apache logs. It looks abnormal to me but I regularly see such things in logs mostly from Windows systems.

> Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 6.0; Trident/4.0; (R1 > 1.6); SLCC1; .NET CLR 2.0.50727; InfoPath.2; OfficeLiveConnector.1.3; OfficeLivePatch.0.0; .NET CLR 3.5.30729; .NET CLR 3.0.30618; > 66760635803; runtime 11.00294; 876906799603; 97880703; 669602703; > 9778063903; 877905603; 89670803; 96690803; 8878091903; 7879040603; > 999608065603; 799808803; 6666059903; 669602102803; 888809342903; > 696901603; 788907703; 887806555703; 97690214703; 66760903; 968909903; > 796802422703; 8868026703; 889803611803; 898706903; 977806408603; > 976900799903; 9897086903; 88780803; 798802301603; 9966008603; > 66760703; 97890452603; 9789064803; 96990759803; 99960107703; > 8868087903; 889801155603; 78890703; 8898070603; 89970603; 89970539603; > 89970488703; 8789007603; 87890903; 877904603; 9887077703; 798804903; > 97890264603; 967901703; 87890703; 97690420803; 79980706603; > 9867086703; 996602846703; 87690803; 6989010903; 977809603; 666601903; > 876905337803; 89670603; 89970200903; 786903603; 696901911703; > 788905703; 896709803; 96890703; 998601903; 88980703; 666604769703; > 978806603; 7988020803; 996608803; 788903297903; 98770043603; > 899708803; 66960371603; 9669088903; 69990703; 99660519903; 97780603; > 888801803; 9867071703; 79780803; 9779087603; 899708603; 66960456803; > 898706824603; 78890299903; 99660703; 9768079803; 977901591603; > 89670605603; 787903608603; 998607934903; 799808573903; 878909603; > 979808146703; 9996088603; 797803154903; 69790603; 99660565603; > 7869028603; 896707703; 97980965603; 976907191703; 88680703; 888809803; > 69690903; 889805523703; 899707703; 997605035603; 89970029803; > 9699094903; 877906803; 899707002703; 786905857603; 69890803; > 97980051903; 997603978803; 9897097903; 66960141703; 7968077603; > 977804603; 88980603; 989700803; 999607887803; 78690772803; > 96990560903; 98970961603; 9996032903; 9699098703; 69890655603; > 978903803; 698905066803; 977806903; 9789061703; 967903747703; > 976900550903; 88980934703; 8878075803; 8977028703; 97980903; > 9769006603; 786900803; 98770682703; 78790903; 878906967903; > 87690399603; 99860976703; 796805703; 87990603; 968906803; > 967904724603; 999606603; 988705903; 989702842603; 96790603; 99760703; > 88980166703; 9799038903; 98670903; 697905248603; 7968043603; 66860703; > 66860127903; 9779048903; 89670123903; 78890397703; 97890603; 87890803; > 8789030603; 69990603; 88880763703; 9769000603; 96990203903; > 978900405903; 7869022803; 699905422903; 97890703; 87990903; 878908703; > 7998093903; 898702507603; 97780637603; 966907903; 896702603; > 9769004803; 7869007903; 99660158803; 7899099603; 8977055803; 99660603; > 7889080903; 66660981603; 997604603; 6969089803; 899701903; 9769072703; > 666603903; 99860803; 997608803; 69790903; 88680756703; 979805677903; > 9986047703; 89970803; 66660603; 96690903; 8997051603; 789901209803; > 8977098903; 968900326803; 87790703; 98770024803; 697901794603; > 69990803; 887805925803; 968908903; 97880603; 897709148703; > 877909476903; 66760197703; 977908603; 698902703; 988706504803; > 977802026603; 88680964703; 8878068703; 987705107903; 978902878703; > 8898069803; 9768031703; 79680803; 79980803; 669609328703; 89870238703; > 99960593903; 969904218703; 78890603; 9788000703; 69690630903; > 889800982903; 988709748803; 7968052803; 99960007803; 969900800803; > 668604817603; 66960903; 78790734603; 8868007703; 79780034903; > 8878085903; 976907603; 89670830803; 877900903; 969904889703; > 7978033903; 8987043903; 99860703; 979805903; 667603803; 976805348603; > 999604127603; 97790701603; 78990342903; 98770672903; 87990253903; > 9877027703; 97790803; 877901895603; 8789076903; 896708595603; > 997601903; 799806903; 97690603; 87790371703; 667605603; 99760303703; > 97680283803; 788902750803; 787909803; 79780603; 79880866903; > 9986050903; 87890543903; 979800803; 97690179703; 876901603; 699909903; > 96990192603; 878904903; 877904734903; 796801446903; 977904803; > 9887044803; 797805565603; 98870789703; 7869093903; 87790727703; > 797801232803; 666604803; 9778071903; 9799086703; 6969000903; 89670903; > 8799075903; 897708903; 88680903; 97980362603; 97980503903; > 889803256703; 88980388703; 789909376803; 69690703; 6969025903; > 89970309903; 96690703; 877901847803; 968901903; 96690603; 88680607603; > 7889001703; 789904761803; 976807703; 976902903; 878907889703; > 9897014903; 896707046603; 696909903; 666603998903; 969902703; > 79680421803; 9769075603; 798800192703; 97990903; 9689024903; > 668604803; 969908671903; 9996094703; 69990642703; 97890895903; > 977805619903; 79980859903; 88980443803; 98970649603; 997602703; > 888802169903; 699907803; 667602028803; 786903283903; 997607703; > 969909803; 798809925903; 9976045603; 97790903; 9789001903; 966903603; > 9789069603; 968906603; 6989091803; 896701603; 6979059803; 978803903; > 997606362603; 88980803; 98970803; 88880921703; 8997065703; 899700703; > 698908703; 797801027903; 7889050903; 87890603; 78690703; 99660069703; > 97980309903; 976800603; 666606803; 898707703; 79880019803; > 66960250803; 7978049803; 88780602603; 79680903; 88880792703; 96990903; > 667608603; 87790730903; 98970903; 9699032903; 8987004803; 88880703; > 89770046603; 978800803; 969908903; 9798022603; 696901903; 799803703; > 989703703; 668605903; 79780903; 998601371703; 796803339703; > 87890922603; 898708903; 9966061903; 66960891903; 96790903; 8779050803; > 98870858803; 976909298603; 9887029903; 669608703; 979806903; > 878903803; 99960703; 9789086703; 979801803; 66960008703; 979806830803; > 99760212703; 786906603; 797807603; 789907297703; 96990703; 786901603; > 796807766603; 896702651603; 789902585603; 66660925903; 9986085703; > 66960302703; 69890703; 789900703; 89970903; 9679060703; 9789002903; > 979908821603; 986708140803; 976809828703; 7988082803; 79680997903; > 99960803; 9788081903; 979805703; 787908603; 66960602803; 9887098703; > 978803237703; 888806804603; 999604703; 977904703; 966904635703; > 97680291703; 977809345603; 8878046703; 988709803; 976900773603; > 989703903; 88780198603; 87790603; 986708703; 78890604703; 87790544803; > 976809850903; 887806703; 987707527603; 79880803; 9897059603; > 897709820603; 97880804803; 66960026703; 9789062803; 9867090803; > 669600603; 8967087703; 78890903; 89770903; 97980703; 976802687603; > 66860400803; 979901288603; 96990160903; 99860228903; 966900703; > 66760603; 9689035703; 9779064703; 7968023603; 87890791903; > 98770870603; 9798005803; 6969087903; 9779097903; 6979065703; > 699903252603; 79780989703; 87690901803; 978905763903; 977809703; > 97790369703; 899703269603; 8878012703; 78790803; 87690395603; > 8888042803; 667607689903; 8977041803; 6666085603; 6999080703; > 69990797803; 88680721603; 99660519803; 889807603; 87890146703; > 699906325903; 89770603; 669608615903; 9779028803; 88880603; 97790703; > 79780703; 97680355603; 6696024803; 78790784703; 97880329903; > 9699077703; 89870803; 79680227903; 976905852703; 8997098903; > 896704796703; 66860598803; 9897036703; 66960703; 9699094703; > 9699008703; 97780485903; 999603179903; 89770834803; 96790445603; > 79680460903; 9867009603; 89870328703; 799801035803; 989702903; > 66960758903; 66860150803; 6686088603; 9877092803; 96990603; 99860603; > 987703663603; 98870903; 699903325603; 87790803; 97680703; 8868030703; > 9799030803; 89870703; 97680803; 9669054803; 6979097603; 987708046603; > 999608603; 878904803; 998607408903; 968903903; 696900703; > 977907491703; 6686033803; 669601803; 99960290603; 887809169903; > 979803703; 69890903; 699901447903; 8987064903; 799800603; 98770903; > 8997068703; 967903603; 66760146803; 978805087903; 697908138603; > 799801603; 88780964903; 989708339903; 8967048603; 88880981603; > 789909703; 796806603; 977905977603; 989700603; 97780703; 9669062603; > 88980714603; 897709545903; 988701916703; 667604694903; 786905664603; > 877900803; 886805490903; 89970559903; 99960531803; 7998033903; > 98770803; 78890418703; 669600872803; 996605216603; 78690962703; > 667604903; 996600903; 999608903; 9699083803; 787901803; 97780707603; > 787905312703; 977805803; 8977033703; 97890708703; 989705521903; > 978800703; 698905703; 78890376903; 878907703; 999602903; 986705903; > 668602719603; 979901803; 997606903; 66760393903; 987703603; > 78790338903; 96890803; 97680596803; 666601603; 977902178803; > 877902803; 78790038603; 8868075703; 99960060603)

Since it's for database purposes and there is no practical limit i'd go for a UserAgents Table with UserAgentId as Int and UserAgentString as NVarChar(MAX) and use a foreign key on the original table.

How's this for big?:

> Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 5.1; Trident/4.0; YPC > 3.2.0; SearchSystem6829992239; SearchSystem9616306563; SearchSystem6017393645; SearchSystem5219240075; > SearchSystem2768350104; SearchSystem6919669052; > SearchSystem1986739074; SearchSystem1555480186; > SearchSystem3376893470; SearchSystem9530642569; > SearchSystem4877790286; SearchSystem8104932799; > SearchSystem2313134663; SearchSystem1545325372; > SearchSystem7742471461; SearchSystem9092363703; > SearchSystem6992236221; SearchSystem3507700306; > SearchSystem1129983453; SearchSystem1077927937; > SearchSystem2297142691; SearchSystem7813572891; > SearchSystem5668754497; SearchSystem6220295595; > SearchSystem4157940963; SearchSystem7656671655; > SearchSystem2865656762; SearchSystem6520604676; > SearchSystem4960161466; .NET CLR 1.1.4322; .NET CLR 2.0.50727; Hotbar > 10.2.232.0; SearchSystem9616306563; SearchSystem6017393645; SearchSystem5219240075; SearchSystem2768350104; > SearchSystem6919669052; SearchSystem1986739074; > SearchSystem1555480186; SearchSystem3376893470; > SearchSystem9530642569; SearchSystem4877790286; > SearchSystem8104932799; SearchSystem2313134663; > SearchSystem1545325372; SearchSystem7742471461; > SearchSystem9092363703; SearchSystem6992236221; > SearchSystem3507700306; SearchSystem1129983453; > SearchSystem1077927937; SearchSystem2297142691; > SearchSystem7813572891; SearchSystem5668754497; > SearchSystem6220295595; SearchSystem4157940963; > SearchSystem7656671655; SearchSystem2865656762; > SearchSystem6520604676; SearchSystem4960161466; .NET CLR > 3.0.4506.2152; .NET CLR 3.5.30729)

There is no stated limit, only the limit of most HTTP servers. Keeping that in mind however, I would implement a column with a reasonable fixed length (use Google to find a list of known user agents, find the largest and add 50%), and just crop any user agent that is too long - any exceptionally long user agent is probably unique enough even when cropped, or is the result of some kind of bug or "hack" attempt.

I got this user agent today, overflowing our vendor's storage field:

> Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 5.1; Trident/4.0; GTB6; > .NET CLR 1.1.4322; .NET CLR 2.0.50727; .NET CLR 3.0.04506.30; MDDR; > OfficeLiveConnector.1.3; OfficeLivePatch.0.0; .NET CLR 3.0.4506.2152; > .NET CLR 3.5.30729)

Ridiculous! 229 chars?

So take that size, double it, double it again, and you should be set until Microsoft's next blunder (maybe this time next year).

Go bigger than 1000!

Assume the user agent string has no limit on its length and prepare to store such a value. As you've seen, length is unpredictable.

In Postgres, there's a text type that accepts strings of unlimited length. Use that.

Most likely though, you'll have to start truncating at some point. Call it good at a reasonably useful increment (200, 1k, 4k) and throw away the rest.

I'll give you the standard answer:

Take the largest possible value you can possibly imagine it being, double it, and that's your answer.

Here is one that is 257

> Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 5.1; Trident/4.0; GTB6; > .NET CLR 1.1.4322; .NET CLR 2.0.50727; .NET CLR 3.0.04506.30; > InfoPath.2; .NET CLR 3.0.04506.648; OfficeLiveConnector.1.3; > OfficeLivePatch.0.0; .NET CLR 3.0.4506.2152; .NET CLR 3.5.30729)

Not an indication of how big a user agent can get, as there's plenty of answers showing the edge cases they've came across, but the longest that could find on http://www.useragentstring.com/pages/useragentstring.php?name=All was 250 bytes.

> Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 6.1; Trident/4.0; SLCC2; .NET CLR 2.0.50727; .NET CLR 3.5.30729; .NET CLR 3.0.30729; Media Center PC 6.0; Media Center PC 5.0; SLCC1; OfficeLiveConnector.1.5; OfficeLivePatch.1.3; .NET4.0C; Lunascape 6.3.